1. Field of the Invention
This invention relates to the field of electrochemistry, and more particularly relates to thermally activated electrochemical cells having a low melting nitrate electrolyte containing novel cathode materials, resulting in an increase in cell potential.
2. Description of the Prior Art
Thermally activated electrochemical cells or batteries have been used quite extensively in military applications, such as a power source for arming devices, because of their long shelf life and compactness, and capability of withstanding shock and vibration. Batteries of this type typically include an electrolyte which, under normal storage conditions, is solid and does not conduct electricity. When the battery and/or the electrolyte is heated to a predetermined temperature, as by igniting a built-in pyrotechnic heat source such as an electric match, squib or percussion primer, the electrolyte, upon changing to a molten state, becomes conductive and ionically connects the electrodes to provide the desired electromotive force.
Nitrate salts have been proposed for use in thermal batteries because of their low melting points. See U.S. Pat. No. 4,260,667 to Miles and Fletcher. For example, potassium nitrate-lithium nitrate (KNO.sub.3 -LiNO.sub.3) mixtures melt at temperatures as low as 124.degree. C. The use of a lower melting electrolyte can shorten a battery's activation time and reduce the weight of heat sources and insulation.
A particular problem area of thermal battery cells is the lack of high performance cathode materials. Adding silver salts to electrolytes as cathode materials to improve cell potentials has previously been unsuccessful because the cathodic reactions involve the reduction of silver ions to the free metal in reversible electrode reactions, such as EQU AgNO.sub.3 +e-.revreaction.Ag+NO.sub.3
Consequently, no net cathodic current can flow in such cells at cathode potentials more positive than the silver ion/silver reversible potential. Additionally, the added silver salts migrate and diffuse to the anode and form silver metal films on the anode surface that interfere with cell operation. Many divalent and trivalent metal ions cannot be used at high temperatures in molten nitrate electrolyte cells since they react rapidly with the nitrate melt to form the metal oxide.
U.S. Pat. No. 4,416,958 to Miles and Fletcher discloses a thermally activated electrochemical cell having a low melting point electrolyte. The electrolyte is composed of a layer of a mixture of lithium perchlorate and lithium nitrate adjacent the anode and of a layer of a mixture of lithium perchlorate, lithium nitrate, and silver nitrate adjacent to the cathode of the cell.
The article "Cyclic Voltammetric Studies of Nitrato Complexes of Various Metal Ions in Molten LiNO.sub.3 +KNO.sub.3 at 180.degree. C." by M. H. Miles et al, J. Electoanal Chem., 221 (1987) 115-128, discloses addition of various metal ions such as Co.sup.++, Cu.sup.++, Au.sup.+++, Mn.sup.++, La.sup.+++, and Ce.sup.+++ ions to molten nitrates, and the effect of such additions.
However, the above article does not disclose or teach the application of the principles or concepts that are disclosed therein to thermal batteries, particularly employing lithium or calcium anodes.
One object of the invention accordingly is the provision of an improved thermal electrochemical cell.
Another object is to provide a novel thermal electrochemical cell utilizing low melting nitrate electrolytes.
A still further object is the provision of improved thermal electrochemical cells incorporating certain cathode materials in the electrolyte.
Yet another object is to provide thermal electrochemical cells having nitrate electrolyte and containing certain metal salts as cathodic material, to increase the potential of the cell.